1. Field of the Invention
This invention relates generally to methods for obtaining subsurface geological information in the exploration of petroleum deposits. More particularly, the invention relates to a method for forecasting formation pore-pressure while drilling.
2. Discussion of the Related Art
Well planning is perhaps the most demanding aspect of drilling engineering. It requires the integration of engineering principles, corporate or personal philosophies, and experience factors. Although well planning methods and practices may vary within the drilling industry, the end result should be a safely drilled, minimum-cost bore hole that satisfies the reservoir engineer's requirements for oil and gas production.
The formation, or pore pressure encountered by the well significantly affects the well plan. If pressure is not properly evaluated, it can lead to drilling problems such as lost circulation, blowouts, stuck pipe, hole instability, and excessive costs. Unfortunately, formation pressures can be very difficult to quantify precisely where unusual, or abnormal, pressures exist.
Formation pore pressure is defined as the pressure exerted by the formation fluids on the walls of the rock pores. The fluids are typically gas, oil, or salt water. The pore pressure supports part of the weight of the overburden (weight of the overlying rock matrix and pore fluid), while the other part is supported by the grains of the rock. The terms pore pressure, formation pressure and fluid pressure are synonymous, referring to formation pore pressure.
Formations are classified according to the magnitude of their pore pressure gradients. In general, two types of formation pressure are recognized. A formation is said to be normally pressured when its pore pressure is equal to the hydrostatic pressure of a full column of formation water. Normal pore pressure is usually on the order of 0.465 pounds per square inch per foot (psi/ft; 0.105 bar per meter, (bar/m)). Abnormal formation pressure exists in zones which are not in direct communication with adjacent strata. The boundaries of the abnormally pressured zone are impermeable, preventing fluid communication and making the trapped fluid support a larger proportion of the overburden stress. Abnormal formation pressures may be as high as 1 psi/ft for tectonically relaxed areas and 0.8 psi/ft for tectonically active areas. Details on the origins and causation of formation pore pressure are beyond the scope of this section and will not be discussed further.
In the past, abnormal formation pressure has been estimated using surface seismic reflection surveys. Such surveys have been used to establish the top of abnormally pressured formations and to evaluate the magnitude of the pressures. Typically, an acoustic source located along the surface is actuated to produce energy in the subsurface in the form of compressional waves. The time required for the energy to travel to a reflector and back to a sensor is measured. The average velocity is computed from the following expression: EQU V =X.sup.2 /t.sup.2 V =X/t
where V is the average velocity, X is the distance traveled by the signal, and t is the time to travel the distance.
The depth of the reflector may be determined by taking the product of the average velocity and one-half the travel time. The interval velocity from seismic profiles is the reciprocal of interval travel time. The reciprocated values can be plotted versus depth to indicate the presence of abnormal pressures. A normal environment exhibits decreasing porosity as compaction occurs. Therefore, the travel times should decrease. An abnormal pressure zone has greater-than-normal porosities for the specific depth and causes higher travel times.
Log analysis is a common procedure for pore pressure estimation in both offset wells and the actual well drilling. New measurement-while-drilling (MWD) tools implement log analysis techniques in a real-time drilling mode. The analysis techniques use the effect of the abnormally high porosities on rock properties such as electrical conductivity, sonic travel time, and bulk density.
The resistivity log was originally used in pressure detection. The log's response is based on the electrical resistivity of the total sample, which includes the rock matrix and the fluid-filled porosity. If a zone is penetrated that has abnormally high porosities (and associated high pressures), the resistivity of the rock will be reduced due to the greater conductivity of water than rock matrix. Upon penetrating an abnormal pressure zone, a deviation or divergence will be noted. The degree of divergence is used to estimate the magnitude of the formation pressures.
Hottman and Johnson developed a technique based on empirical relationships whereby an estimate of formation pressures could be made by noting the ratio between the observed and normal rock resistivities. Subsequent to the Hottman and Johnson approach, unpublished techniques were developed that used overlay or underlay for a quick evaluation of formation pressures. The overlay (underlay) contains a series of lines that represent formation pressure expressed as mud weight. The overlay (underlay) is shifted left and right until the normal pressure line is aligned with the normal trend. Formation pressures are read directly from a visual inspection of the location of the resistivity plots within the framework of the lines.
The Hottman and Johnson procedure, as well as the overlay techniques, assume that formation resistivities are a function of lithology, fluid content, salinity, temperature, and porosity. The procedures make the following assumptions with respect to these variables: lithology is shale, shale is water filled, water salinity is constant, temperature gradients are constant, and porosity is the only variable affecting the pore pressure. Formations with varying water salinities can prevent the reliable use of the Hottman and Johnson technique.
The sonic log has been used successfully as a pressure evaluation tool. The technique utilizes the difference in travel time between high-porosity overpressure zones and low-porosity, normal pressure zones. Hottman and Johnson studied several wells and developed a pressure relationship. The manner in which formation pressures are calculated using the Hottman and Johnson approach is similar to their method for resistivity plots. Observed transit times are plotted, and the normal trend line is established and extrapolated throughout the pressure region. At the depth of interest, the difference between the observed and normal travel times is established. This difference is used to estimate the formation pressure by correlating with equivalent mud weight in pounds per gallon.
Although each of the above logging techniques is relatively effective in determining the formation pressure, each is post determinative. That is to say that the well must be drilled before the calculation is made. Thus the risk of a blowout or major kick has already occurred and much of the safety concerns have passed.